1. Field of the Invention
This invention resides in the fields of metallic glasses and nanotechnology, and particularly in methods for strengthening metallic glasses by the incorporation of crystalline particles in the glassy matrix.
2. Description of the Prior Art
Metallic glasses are known to be superior to conventional metals by virtue of the improved mechanical properties of the glasses, including their higher tensile strength, fatigue strength, hardness, axial fatigue, and fracture toughness. These qualities, combined with a mid-range density, have resulted in metallic glasses being used for certain high-performance and high-impact applications. Examples of products that have been manufactured from metallic glasses are aeronautical and industrial turbo-engines, airframes, knives, golf-club heads, and even wristwatches.
Metallic glasses are alloys that have an amorphous microstructure. The amorphous state is achieved by cooling the alloy composition from a melt at a cooling rate that is fast enough to avoid crystallization. In the early investigations of metallic glasses, the alloys that were used required cooling rates of 104–106 degrees Celsius per second (also designated as “K/s”) to bypass crystallization. Because of this requirement, cooling could only be performed on bodies of the melt with very small dimensions, such as layers less than 100 microns in thickness or small droplets, and the amorphous material was produced only in the form of thin ribbons or fine powders.
Subsequent investigations have led to the development of several families of alloys that can be cooled to an amorphous form at much slower cooling rates, such as 103 K/s or less, and most recently cooling rates within the range of 0.1–100 K/s. Among the leading investigators in this development are A. Peker, W. L. Johnson, and A. Inoue, whose investigations are reported in the literature, notably in Peker, A., and W. L. Johnson, Appl. Phys. Lett. 63, 2342 (1993), and U.S. Pat. No. 5,288,344 (Peker, A., and Johnson, W. L., assigned to California Institute of Technology), issued Feb. 22, 1994, A. Inoue, notably in Inoue, A., et al., “Fabrication of Bulky Zr-Based Glassy Alloys by Suction Casting into Copper Mold,” Materials Transaction, Japan Institute of Metals (English Version), vol. 36, no. 9, pp. 1184–1187 (1995), and Inoue, A., et al., “Preparation and Thermal Stability of Bulk Amorphous Pd40Cu30Ni10P20 Cylinder of 72 mm in Diameter,” Materials Transaction, Japan Institute of Metals (English Version), vol. 38, no. 2, pp. 179–183 (1997). The contents of these and all other literature and patent citations in this specification are incorporated herein by reference.
Alloys that can form amorphous solids at these low cooling rates have led to the emergence of a class of metallic materials known as bulk metallic glasses (BMGs) since the lower critical cooling rate permits these glasses to be produced in dimensions of several centimeters. By virtue of this flexibility, BMGs are suitable for many structural and functional applications, including the larger-scale products among those listed above and components in general for the defense industries, manufacturing industries, and recreational products, as well as magnetic materials, medical instruments, and implants.
An even more recent development in BMGs is the discovery that the mechanical properties of the glasses, and the magnetic properties of those used as magnets, can be further enhanced by the dispersion of crystallites throughout the amorphous matrix of the glass. Crystallites with sizes both in the nano-scale and the micro-scale have been investigated. Methods of achieving these dispersions, particularly of nanocrystallites, and the benefits that the dispersions offer are described for example in Perepezko, J. H., et al. (Wisconsin Alumni Research Foundation), U.S. Pat. No. 6,261,386 B1 (issued Jul. 17, 2001). The methods generally consist of controlled cooling techniques that result in partial crystallization (devitrification) of the amorphous material. The crystals are thus grown by nucleation, however, which is generally accompanied by grain growth, and as noted by Perepezko et al., the quality of the resulting dispersion, particularly the number of crystals formed and their size and distribution throughout the amorphous matrix, are difficult to control. The crystals reported by Perepezko et al. are in the nano-range, i.e., 100 nm or less in diameter, which are of particular interest where a high-density dispersion is sought. To achieve nanocrystalline dispersions, the Perepezko et al. disclosure proposes the seeding of the amorphous matrix with elements that are insoluble in the amorphous matrix. This however involves the introduction of foreign matter into the melt, and requires control of the seed size, while still causing grain growth. The formation of and benefits offered by micro-range crystals are reported by Hays, C. C., et al., Phys. Rev. Lett. 84: 2901–4 (2000). The crystals reported in this paper are dendrites that improve the toughness and plasticity of the glasses.